This is a standard file format for SGI workstations. Images can be greyscale, 24-bit color, or 48-bit color. The format formerly allowed 8-bit color images, but this is now obsolete (and is not supported by the SGI decode plug-in). SGI files can optionally use an RLE compression scheme which is effective only for images with blocks of solid color.
Files in this format are rarely encountered by PC users.